1. Field of the Invention
The present invention relates to a device for sensing an oxygen concentration in a gaseous body, such as an exhaust gas of an internal combustion engine, and also relates to an air/fuel ratio feedback control system using an oxygen concentration sensing device.
2. Description of Background Information
Air-fuel ratio feedback control systems for an internal combustion engine are becoming generally used, which are constructed such that the oxygen concentration in the exhaust gas of the engine is detected by an oxygen concentration sensor and an air-fuel ratio of a mixture to be supplied to the engine is feedback controlled in response to a result of the detection of the oxygen concentration so as to purify the exhaust gas and improve the fuel economy.
As an example of oxygen concentration sensing device for use in the air-fuel ratio control system of the above mentioned type, an oxygen concentration sensing device having an output signal whose level is proportional to the oxygen concentration in a measuring gas (whose oxygen concentration is to be measured) is described in Japanese patent application laid open No. 58-153155. This oxygen concentration sensing device has a sensor element which has general construction including a pair of flat solid electrolyte members having oxygen ion permeability. These oxygen-ion conductive solid electrolyte members operative as active plates are placed in the atmosphere of the oxygen-containing measuring gas. Further, two electrodes are provided on the front and back surfaces of both of the solid electrolyte members. In other words, each pair of electrodes sandwich each solid electrolyte member. These two solid electrolyte members each having a pair of electrodes are arranged in face to face relation with each other to form a gap portion, or in other words, a restricted region between them.
With this arrangement, one of the solid electrolyte members serves as an oxygen pump element and the other one of the solid electrolyte members serves as a sensor cell element for sensing an oxygen concentration ratio. In the atmosphere of the mesuring gas, a drive current is supplied across the electrodes of the oxygen pump element in such a manner that the electrode facing the gap portion is used as a negative electrode. By the supply of this current, the oxygen component of the gas within the gap portion is ionized on the surface of the negative electrode of the solid electrolyte member operating as the oxygen pump element. The oxygen ions migrate through the inside of the oxygen pump element to the positive electrode, where the oxygen ions are released from the surface of the positive electrode in the form of the oxygen gas.
While this movement of oxygen ions is taking place, a voltage is generated across the electrodes of the solid electrolyte member operating as the sensor cell element because the oxygen concentration is different for the gas in the gap portion and the gas outside the electrodes of the sensor cell element. This difference of the oxygen concentration is caused by a reduction of the oxygen gas component within the gap portion. Then, if the magnitude of the electric current supplied to the sensor cell element is varied so as to maintain the voltage across the sensor cell element, the magnitude of the electric current varies substantially linearly in proportion to the oxygen concentration of the test gas at a constant temperature.
In this type of oxygen concentration sensing devices, if an excessive current is supplied to the oxygen pump element, it causes the so called blackening phenomenon by which the oxygen ions are removed from the solid electrolyte members. For instance, when zirconium dioxide (ZrO.sub.2) is utilized as the solid electrolyte, the oxygen ions O.sub.2 are removed from the zirconium dioxide (ZrO.sub.2) so that zirconium (Zr) is separated out. As a result of this blackening phenomenon, deterioration of the oxygen pump element takes place rapidly, to cause a debasement of an operation of the oxygen concentration sensing device as a whole.
FIG. 1 shows curves indicating the relationship between the oxygen concentration and the magnitude of the pump current supplied to the oxygen pump element for different values of the voltage Vs generated by the sensor cell element which is expressed as a parameter. A region of occurence of the blackening phenomenon is also illustrated in FIG. 1. The boundary of the region of blackening phenomenon is, like the curves of the pump current which is expressed by using the parameter of the voltage of the sensor cell element Vs, on a curve linearly increases with respect to the oxygen concentration. This means, whether or not the magnitude of the current supplied to the oxygen pump element is in the region of blackening phenomenon is determined by means of the voltage Vs generated by the sensor cell element. Therefore, it can be assumed that magnitude of the current supplied to the oxygen pump element approaches to the region of the blackening phenomenon when the voltage Vs is higher than a predetermined level, and the occurence of the blackening phenomenon can be prevented by reducing the current to the oxygen pump element.
In air/fuel ratio control systems using this type of oxygen concentration sensing device, magnitude of the current to be supplied to the oxygen pump element is set at a level below a critical level of the occurence of the blackening phenomenon in order to prevent the said phenomenon. Therefore, by comparing the output signal level of the oxygen concentration sensing device with a reference voltage, detection is performed as to whether the air/fuel ratio of mixture is on the rich side or the lean side with respect to the target air fuel ratio. If the air/fuel ratio control system is of the type in which the air/fuel ratio is controlled by the supply of the secondary air, the secondary air is supplied when the rich air/fuel ratio is detected, and the supply of the secondary air is stopped when the lean air/fuel ratio is detected. In this way, the air/fuel ratio of mixture to be supplied to the engine is controlled toward the target air/fuel ratio.
The feedback control of the air/fuel ratio may be stopped in response to operating conditions of the engine. For instance, when the engine load is high, or when the engine coolant temperature is low, it is general to stop the feedback control of the air/fuel ratio. The air/fuel ratio may be enriched, for example by a fuel increasing system, when the feedback control of the air/fuel ratio is stopped. In such a period in which the feedback control is stopped, a critical level of the occurence of the blackening phenomenon reduces as the air/fuel ratio becomes rich. Therefore, the blackening phenomenon inevitably occurs unless the supply of the pump current is stopped in such a period.
On the other hand, for supplying the current to the oxygen pump element, there is a current supply circuit in which the magnitude of the current to the oxygen pump element is controlled in response to a result of a comparison between the voltage generated by the sensor cell element and a reference voltage.
FIGS. 2A and 2B show the variation of the control voltage supplied to the constant current circuit and the corresponding variation of the current supplied to the oxygen pump element in a conventional arrangement. As shown in FIG. 2A, when the supply of the control voltage to the constant current circuit is initiated, for instance, at the time of start of the engine, the constant current circuit starts to supply the pump current to the oxygen pump element. However, due to a delay of response of the air/fuel ratio control system, the pump current does not reach a desired constant level immediately. Instead, as shown in FIG. 2B, an overshoot of the pump current occurs during a transitional period. Therefore, the magnitude of the pump current exceeds the critical level of the occurence of the blackening phenomenon so that the blackening phenomenon may actually take place.
In the other aspect, because of the presence of the gap portion between the oxygen pump element and the sensor cell element, delay of response of the sensor cell element inevitably arises. Particularly, the level of the output signal does not increase and reach the reference voltage immediately, even the pump current to the oxygen pump element has risen above the constant current value corresponding to the reference current value after the start of the supply of the pump current. Instead, the output signal level increases gradually as illustrated in FIG. 2C.
For this reason, although the output signal level of the sensor cell element is monitored for detecting an overcurrent flowing through the oxygen pump element in some systems, it has been difficult to prevent the generation of an overcurrent immediately after the start of the supply of the pump current to the oxygen pump element.